Epigenetics, that over or above genetics, refers to hereditary changes in genome expression that do not involve alteration of DNA sequences. Epigenetics is a study for physiological phenotypic trait variations that are caused by external or environmental factors that switch genes on and off. Hence, improvement of epigenetic research relies on a wide range of gene editing technology.
The gene editing technology is the most powerful tool to insert, replace, and delete targeted DNA from genome. DNA sequence-specific recombination has been widely used for the gene editing technology to regulate genetic modifications, such as conditional gene expression, conditional mutagenesis, gene replacement and chromosome engineering in mammalian. There are several engineered nucleases being used: Transcription Activator-Like Effector Nucleases (TALENs), CRISPER/Cas9 system, Sleeping Beauty, PiggyBac, Cre/LoxP system, and Flp/Frt systems.
Cre-mediated recombination has been widely used to manipulate the genomes of mammalian and non-mammalian organism. The Cre (Cyclization Recombinase) derived from bacteriophage P1 recognizes LoxP sites, which is comprised of 34 base pair sequences. A segment of DNA, which is flanked by the LoxP sites, is deleted by the Cre mediated recombination. The manipulation of the mouse genome has been enabled to access by the Cre/LoxP system. A common application of the Cre/LoxP system is to create conditional knockouts in mice. LoxP sites are introduced into the germ line. The mice are mated with a strain that expresses Cre in a tissue or developmentally restricted manner causing recombination of floxed gene to occur only in specific tissues or at specific times in development.
The site-specific recombination has also been used to manipulate mammalian chromosome, to insert exogenous DNA at specific sites in the genome, to promote activity of specific genes, and to suppress activity of specific genes. However, spatial- and temporal-controlled gene activation or deletion is often hampered by difficulties expressing the recombinase in the cells at the desired time and place. Plasmid and viral expression vectors are frequently used; however, the efficiency of DNA-mediated gene transfer is low. In addition, the early gene disruptions during embryogenesis by tissue-specific Cre expression in Cre Knock-in mice may cause abnormal development that leads to embryonic lethality. This fetal problem results in the limitation to study in terminally differentiated cells.